Label-free optical biological/chemical sensors are essential in the application of medical diagnosis, healthcare and environmental monitoring, amongst others. Among all the approaches, silicon optical microresonator-based biosensors are regarded to be very promising due to their high sensitivity to refractive index change (˜10−4-10−7 RIU (refractive index unit)), comparable to the conventional surface plasmon resonance (SPR) technique sensitivity), compact footprint (˜10's μm-˜100 μm), and potential large-scale-integration with microfluidics.
Single microring resonator-based biosensors in silicon-on-insulator (SOI) have been demonstrated using either conventional microrings [K. De Vos, et al., Opt. Express 15, pp. 7610-7615 (2008); D. Xu, et al., Opt. Express 16, pp 15137-15148 (2008); M. Iqbal, et al., IEEE J. Select. Top. Quantum Electron. 16, pp. 654-661 (2010).] or slot-waveguide microring [T. Claes, et al., J. Photon. 1, pp. 197-204 (2008).]. The demonstrated detection limit ranges from 10−5-10−7 RIU.
However, for nearly all the demonstrated microresonator sensors, the wavelength-scanning method using wavelength-tunable lasers was considered to be the “default” technique for measuring sharp resonance wavelength shift. The wavelength-scanning method requires high-resolution wavelength tunable lasers in order to measure the sharp resonance wavelength shift, in which the detection limit is limited by the laser resolution. Furthermore, high-resolution wavelength-scanning lasers are very expensive and not suitable for point-of-care applications.